It is necessary to heat workpieces for various processes utilized in treating the workpieces, such as for example, processes utilized for hardening the outer surfaces of the workpieces. Workpieces have been heated heretofore by a fluidizing process in which the workpieces have been immersed in finely divided particles or particulate material in a fixed container and a gas passed through the finely divided particles to provide fluidizing of the particles about the workpieces for effecting heating of the workpieces in the fixed container. The fluidizing of the particles causes a random movement of the particles and a rubbing action of the particles against the outer surface of the workpieces to effect a transfer of heat between the particles and the workpieces. The utilization of very fine particles provides a large surface area for heating and the heat is normally provided from the wall of the fixed container in which the particles and workpieces are positioned for the heat treatment.
Heretofore, workpieces have also been positioned within a container having abrasive material therein for contacting the workpieces with the container being rotated in a tumbling action. The rotation of the container causes movement of the abrasive material and workpieces during the tumbling action to provide a desired surface finish to the workpieces. However, the container has not been heated and the abrasive material has not been utilized for transferring heat between the workpieces and the container. Also, abrasive materials utilized in rotating containers heretofore has not been of a sufficiently small particle size such as less than around 800 microns, for fluidizing from rotation of the container to transfer heat effectively between the workpieces and the container.
Gas has been commonly employed in a fixed container for fluidizing particulate material by flowing through the particulate material from the bottom to the top. Advantages of utilizing a fluidized bed for heating of a workpiece such as by treating the outer surface of the workpiece to obtain a hardened outer case include the following: (1) heat transfer is more uniform than in an air furnace; (2) contamination is minimized as the fluidized bed material and gas can be independently controlled; (3) the rate of heating and cooling can be controlled by cycling fluidization action on and off; (4) the furnace can be shut down and restarted without fear of thermal shock; (5) the workpiece can be exposed to a desired gas mixture for precise periods of time and temperature; and (6) the bed can be of materials which are inert to the workpiece so all the reactive elements are provided from the injected gases.